Steel strip hot-rolled from the cast raw material or billets directly in successive process steps has been described in German Patent No. 32 41 745. After the striplike continuous casting is formed and after its complete solidification it is rolled up into a coil at the casting speed and then the castings are separated and removed after reaching the permitted or desired coil weight. Then after an intervening storage in the furnace, the coil is transferred to an unwinding device. There the unwinding of the casting from the coil occurs. Then the unwound casting is fed into the rolling mill for rolling to make the steel strip.
The plant for performing the process includes a continuous casting plant for casting a striplike continuous casting, a winding device forming a coil from the billet following the casting machine, a transverse dividing device provided between the casting machine and the winding device, a storage furnace for the coils, an unwinding device for unwinding the casting to a flat strand and a rolling mill for rolling the unwound casting to a linear hot-rolled strip with this unwinding device.
According to German Patent No. 32 41 745 beneficial prerequisites for economical manufacture of steel strip from cast raw material are provided since the device required in the usual hot rolling of steel strip for reheating the cast slabs is not required and also expensive intermediate conveyor devices and intermediate storage locations are avoided. Also the expensive deformation required for the rolling of conventional cast slabs and the high energy consumption required for that deformation are avoided.
According to German Patent No. 32 41 745 also the problems resulting from the different speeds in casting of the starting or raw materials and in the subsequent rolling are eliminated because rolling the long cast billet in a coil after the casting provides a buffer between the casting unit and the rolling mill. Thus there is no direct dependence of the casting speed and the rolling speed.
Noticeable drawbacks in this process and in the plant have arisen in practice. Not every striplike cast raw material can be wound to a coil directly after its solidification. Therefore a reducing rolling mill must be provided between the casting machine or unit and the winding device with which the casting, at a temperature between the solidification temperature and the usual rolling temperature, is reduced to a cross section suitable for winding and for subsequent unwinding at the rolling temperature.
It is disadvantageous in practice that the cooling of the long cast billet wound to a coil from the solidifying temperature to the rolling temperature does not occur uniformly in a furnace but is more rapid in the outer coil layers of the coiled strip than in the interior layers of the coil.
In the rolling process connected with the unwinding of the long cast billet from the coil the long cast billet has a necessarily variable temperature profile in its longitudinal direction by which the quality of the steel strip made by the subsequent rolling process can be permanently damaged. Since the rolling cycle is comparatively short compared to the unrolling of the casting from the coil, a temperature compensation or balancing in the longitudinal direction of the unwound long cast billet can no longer take place a practical manner.